Issue description

UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.79 Safari/537.36
Steps to reproduce the problem:
1. Open the attached testcase in Chrome.
2. Click/tap into the text <input> so as to focus it.
What is the expected behavior?
The <div> should turn green since the <input>, which is a descendant of the <div>,
has become focused, thus causing the :focus pseudo-class to match the <input>,
which should cause the <div> to match :focus-within,
because it now has a descendant which matches :focus.
What went wrong?
The <div> remained white, indicating that the :focus-within pseudo-class in the webpage's CSS didn't start matching the <div>.
Did this work before? No
Chrome version: 51.0.2704.79 Channel: stable
OS Version: OS X 10.11.5
Flash Version: Shockwave Flash 21.0 r0
Specification: https://drafts.csswg.org/selectors-4/#the-focus-within-pseudo

Since this is a high-level tracking bug, adding the label Objective. Per Blink>CSS bug protocols, for clear responsibility, any concrete work on this issue should take place on a dependent bug that is filed under one component only.

rego@ it's good to know, and feel free to take it :)
BTW, the screenshot in comment#13 was for what I was playing with on Safari TP25.
http://jsbin.com/cokigavero/edit?html,css,output (without <slot>)
http://jsbin.com/tasoxomivu/edit?html,css,output (with <slot>)
Notice the difference that the latter <input> gets green border, as it matches
the :focus-within in the document stylesheet. In the former <input> didn't
get green border, even though it is focused, because it is inside a shadow tree
and the style rule in the document tree didn't reach there.

Common ancestor patch has landed, and I've added a new WPT test in:
https://github.com/w3c/web-platform-tests/pull/5685 (still pending to review).
@rune do you think we're still missing something else?
Or could we mark the Runtime Flag as stable for shipping this in M60?
Thanks for your feedback.

> IIUC, the test is about clearing :focus on elements becoming display:none, so it's not really directly a question about :focus-within as :focus-within is just kept in sync with :focus, right?
Yeah that's true, I've splitted it into 2 different tests. Anyway that should be reviewed and merged first on WPT and then they'll be imported into Blink.
@rune my question about enabling the runtime flag still stands. Are you missing anything else regarding :focus-within before we ship the feature? Thanks.